1. Field of the Invention
The present invention relates to a grid for a lead storage battery. More particularly, the present invention relates to a method for producing a grid having a high deformation (such as grid growth) resistance in a stable manner regardless of the kind of machining such as expansion and punching.
2. Description of the Related Art
A grid for a lead storage battery is produced mainly by casting method or machining method such as expansion and punching.
Machining allows continuous production of grids and installation of a continuous line including filling with paste and drying, and thus gives an extremely high productivity as compared with casting method. Machining also makes it easy to produce a thin grid designed for higher power or reduced weight and to fill the grid with a paste thereafter.
In accordance with machining, a slab (thick plate material) having a thickness of from 10 mm to 30 mm cast or extruded from a lead-calcium-tin alloy is generally rolled sequentially over a plurality of rolls. The sheet thus rolled is once wound. Thereafter, the rolled sheet is machined while being rewound to produce a grid. The reason why the rolled sheet is not directly fed continuously to machining step such as expansion step and subsequent paste filling step but is wound before being fed to machining step or paste filling step while being rewound is that the production speed of rolling is generally higher than the speed of machining and/or paste filling. Further, it is necessary that the suspension of machining step or paste filling which can be possibly caused by some troubles in the process of production of a rolled sheet be avoided. Moreover, when the place of production of a rolled sheet and the place of production of grid and filling the grid with a paste are remote from each other, they cannot be connected.
The conventional method is disadvantageous in that strain developed during machining causes the recrystallization of alloy to proceed, rendering the electrode plate deformable such as grid growth during use in battery. In other words, the foregoing machining method is disadvantageous in that when machining is effected on the rolled sheet, i.e., a blade having a predetermined shape is pressed against the rolled sheet to cut the rolled sheet, the frames forming the grid and the crossover point of frames have strain left therein, causing the recrystallization thereof to proceed and hence making it easy for the electrode plate to elongate remarkably during use in battery. In particular, when such a machined grid is used as a grid for a positive electrode in a lead storage battery, lead on the surface of the grid is converted to lead dioxide, which has a greater volume that causes the grid to be subject to tensile strength. If the recrystallization of the grid has proceeded, the grid exhibits a deteriorated mechanical strength and thus can undergo elongation and deformation under the application of tensile strength. As the elongation or deformation of the grid increases, the adhesivity between the grid and the active material is deteriorated, causing the drop of capacity.
Further, the rolled sheet which has been allowed stand without any treatment after production is disadvantageous in that hardening due to the progress of aging causes a great deterioration of the elongation, resulting in the increase of occurrence of break during expansion. Thus, rolling conditions and treatment conditions have been studied to improve the mechanical properties of a rolled sheet.
As one of approaches for solving the foregoing problems, there has been studied a process involving the storage of a rolled sheet at low temperatures for the purpose of retarding age hardening during storage. However, it is usual that it takes some time for the rolled sheet which has been introduced into the storage chamber to cool. Thus, the age hardening of the alloy can proceed even in a short period of time between the termination of rolling and the lowering of the temperature, causing a great change in the state of the rolled sheet and hence deteriorating the stability thereof during machining.
Further, when the rolled lead alloy sheet which has been produced is put in water so that it is rapidly cooled, the mechanical properties of the sheet after hardening can be improved. This is because when the alloy is rapidly cooled, the additive elements can not diffuse quickly in state from high solubility to low solubility, producing supersaturation that causes the deposition of fine intermetallic compounds in the alloy during age hardening. However, this approach is disadvantageous in that when water is attached to the surface of the sheet, oxides are produced thereon, resulting in the production of an electrode plate from which the active material can easily come off.
It is an object of the present invention to provide a method for producing a grid having an excellent life which can reduce the recrystallization due to strain during machining to inhibit the deformation of grid. It is another object of the present invention to improve the mechanical properties of a rolled sheet to be machined by inhibiting the recrystallization due to age hardening.
According to a first aspect of the present invention, a method for producing a grid for a lead storage battery comprises a first step of sequentially rolling lead alloy over a plurality of rolls; a second step of machining the rolled sheet obtained at the first step to produce a grid for a lead storage battery; and between the first step and the second step, a cooling step to cool the rolled sheet so that (1) the surface temperature thereof is 10xc2x0 C. or lower at least once and (2) the total time during which the rolled sheet is exposed to an atmosphere of higher than 10xc2x0 C. is 10 hours or less. In this manner, recrystallization due to strain during machining can be minimized, making it possible to inhibit the deformation of the grid.
In a second aspect of the present invention, the cooling step is effected in a gas phase after the termination of the first step of the first aspect of the present invention. In this manner, the rolled sheet can be prevented from undergoing recrystallization due to age hardening before machining without wetting on the surface thereof, making it possible to inhibit break during working.
In a third aspect the present invention, which is a further embodiment of the second aspect of the present invention, the cooling step is effected in such a manner that the surface temperature of the rolled sheet lowers to 10xc2x0 C. or lower at a rate of 5xc2x0 C./min or more.
In a fourth aspect of the present invention, the lead alloy is a lead-calcium-tin alloy comprising 0.04% by mass or more, 0.09% by mass or less, 1.0% by mass or more and 2.4% by mass or less, respectively, in the first to third aspects of the present invention.